The 3rd-Generation Partnership Project (3GPP) is continuing development of specifications for the Universal Terrestrial Radio Access Network (UTRAN). That continuing development includes improved support for heterogeneous networks. In heterogeneous networks (also known as HetNets), an existing homogeneous network, e.g., a cellular network comprising several similarly-sized macro cells, is overlaid with additional lower-power cells, such as femtocells, which can be referred to as Home Node B's or HNBs. These femtocells may be configured as home base stations to support connectivity for only a limited set of subscribers, known as a Closed Subscriber Group (CSG).
HNBs may operate in open, closed, or hybrid access modes. In open access mode, the HNB appears as a normal base station to mobile terminals (UEs, in 3GPP terminology), and can freely provide services to non-member mobile terminals. In closed access mode, the HNB only provides services to members of its CSG. In the hybrid access mode, the HNB provides services to both its members and also to non-members, but prioritizes members over non-members.
The Wideband Code Division Multiple Access (WCDMA) HNB architecture has been developing at a very high rate in the last few years. One of the developments concerns improving mobility procedures for UEs moving between HNBs as well as for UEs moving between HNBs and macro cells. Some examples of mobility that are currently supported include the following:                Hard handover (HHO) mobility, based on Radio Access Network Application Part (RANAP) signaling. Hard handovers can be carried out between any type or source or target, including, for example, a source macro Radio Network Controller (RNC) and a target HNB with any sort of cell access mode (closed, hybrid or open). This type of mobility follows procedures specified in the 3GPP standards document entitled “UTRAN Iu interface; Radio Access Network Application Part (RANAP) signaling”, 3GPP TS 25.413, v. 10.2.0 (June 2011), and is based on the principle that access control and member verification processes, which determine whether a mobile terminal should be permitted to enter a closed mode cell or whether a mobile terminal should be prioritized within a hybrid mode cell, are carried out in the core network.        HNB-to-HNB hard handover via the “Iurh” interface, as described in section 5.7 of “UTRAN Architecture for 3G Home Node B (HNB); Stage 2,” 3GPP TS 25.467, v. 10.2.0 (June 2011). This type of mobility is possible between HNBs only and can occur between open access HNBs or between hybrid access HNBs with the same CSG, or between closed access HNBs with the same CSG. This type of mobility occurs without any involvement of the core network, i.e., mobility signaling is handled within the HNB gateway domain without ever exiting such domain.        HNB-to-HNB soft handover via the “Iurh” interface, as per section 5.7.3 of 3GPP TS 25.467, v. 10.2.0. This type of mobility is possible in the same scenarios as for the HNB-to-HNB hard handovers discussed above, with the difference being that mobility happens in a “soft” manner, such that the UE always maintains at least one radio link with the core network, unlike a hard handover, in which old radio links in the UE are removed.        
The 3GPP has recently prioritized a number of mobility scenarios where enhanced mobility procedures are desired. Notably, the only mechanism currently existing to allow mobility between macro RNCs and HNBs is to use the RANAP-based hard handover mobility procedures, as described above, which are carried out via the core network. In these procedures, access control (performed for handover to closed access CSG cells) and membership verification (performed for handover to hybrid CSG cells) are carried out in a centralized manner by the core network, which holds subscriber information. However, this type of mobility is not efficient in terms of performance, due to the fact that every mobility signaling is routed via the CN, generating large delays.